Tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) are widely used as monomers in the manufacture of plastic and elastomeric fluoropolymers. See, for example, J. Scheirs in Modern Fluoropolymers, Wiley, 1996. The worldwide consumption of TFE exceeds 105 tons/year. HFP is used as a comonomer to manufacture thermoplastic and elastomeric fluoropolymers and as starting material for making hexafluoropropene oxide (HFPO). The worldwide consumption is estimated to be 30,000 tons/year.
There are several known methods for manufacturing TFE and HFP. The most common method and almost exclusively used at industrial scale, involves pyrolyzing CHClF2 (R-22). See for example, U.S. Pat. No. 2,551,573. The high temperature (600° C. to 1000° C.) pyrolysis of CHClF2 yields TFE and HFP in high yields. But there are environmental concerns with R-22. This process produces equimolar amounts of aqueous hydrochloric acid and considerable amounts of partially fluorinated and chlorinated compounds, which are difficult to separate from TFE to obtain polymerization grade TFE (U.S. Pat. No. 4,898,645). For the aqueous hydrochloric acid, industrial applications are generally sought that can use the aqueous hydrochloric acid. The fluorinated and other side products have to be incinerated through thermal oxidizers, which is another costly process and produces high amounts of CO2.
U.S. Pat. No. 5,611,896 describes a process where elemental fluorine is reacted with carbon to produce CF4, which is converted to TFE in a plasma torch in the presence of carbon. Unreacted CF4 is fed back to the plasma. Thus, this technology is advantageously “closed-loop” which means emissions to the environment are minimal. But this process is hardly economically viable due to the use of costly elemental fluorine and the high-energy consumption involved.
U.S. Pat. Nos. 5,633,414 and 5,684,218 describe a plasma process, where metal fluorides, particularly CaF2 as a cost efficient fluorine source, are reacted with carbon in a plasma. Thus, the costs for elemental fluorine are avoided. This technology still requires high-energy consumption.
A further method described in the art involves reacting TFE and/or HFP with ethylene and then fluorinating the cyclobutanes by electrochemical fluorination (ECF). This perfluoro-cyclobutane product is then pyrolyzed using conventional pyrolyzing techniques as described for example in EP 455,399 including the references cited therein and WO 00/75092. Any by-products formed in the ECF process are separated off and are not further used in accordance with the teaching of WO 00/75092. Accordingly, substantial waste material is produced with this process, which causes an environmental burden and makes the process economically less attractive. Additionally, the process requires the use of TFE as one of the starting compounds, which creates an additional economical disadvantage as part of the TFE produced is needed to produce further TFE.
U.S. Pat. No. 3,081,245 discloses a process for preparing TFE that comprises feeding a saturated perfluorocarbon to a continuous electric arc, passing the emerging gaseous product through a carbon bed at a temperature of 2700° C. to 2000° C. and quenching the resulting gaseous product mixture to less than 500° C. in less than one second.
EP 371,747 discloses a process for making TFE by heating in the presence of a gas selected from Ar, HF, CO, CF4, and CO2 at a temperature of at least 2000° K. a C2 to C10 compound containing fluorine and hydrogen in which the F to H ratio is greater than or equal to 1 and the F to C ratio is greater than or equal to 1. Heating is carried out with a Direct Current (DC) plasma or through radio frequency energy.
Another chlorine-free process for making TFE is disclosed in GB 766 324 by pyrolyzing a fluorocarbon with at least 3 carbons per molecule. Pyrolyzing occurs at a temperature of at least 1500° C. preferably generated in an electric arc. The side products of the pyrolysis are fed back in the pyrolysis furnace after the separation of TFE. The fluorocarbons to be pyrolyzed are obtained from exhaustive fluorination of petroleum fractions using elemental fluorine, which renders the process economically unattractive.
Still another chlorine-free method to make TFE is described in EP 0 647 607. Finely divided fluoropolymers such as PTFE or perfluoro- or highly fluorinated copolymers are pyrolyzed with superheated steam. The source of this feedstock is scrap material that cannot be used, or materials from worn out equipment. This process is an economical management of waste material. Another chlorine-free process to make TFE is described in WO 01/58840-A2. Solid particulate fluorocarbons, particularly PTFE and highly or perfluorinated polymers are subjected to DC plasma to yield TFE. Still another chlorine-free process to make TFE is disclosed in WO 01/58841-A1 where gaseous or liquid fluorocarbons are pyrolyzed via DC plasma. Another chlorine-free process to make TFE is described in WO 01/58584-A2. Gaseous, liquid, and solid perfluorocarbons, particularly perfluoropolymers, are pyrolyzed via inductive heating. These processes cannot replace the standard technology via R-22, because the technology does not produce new C—F-bonds and therefore cannot meet the demand for TFE.
Thus, the need exists for a process to manufacture TFE and/or HFP that is efficient, environmentally friendly, and/or cost efficient.